1. Field of the Invention
The present invention relates to circuit elements within semiconductor chips. More specifically, the present invention relates to a method and an apparatus for providing a voltage-controlled variable inductor within a semiconductor chip.
2. Related Art
As microprocessor clock speeds continue to increase at an exponential rate, it is becoming progressively harder to generate clock signals accurately enough to synchronize the various components within a microprocessor. Microprocessors presently make use of on-chip clock generation circuitry, which is synchronized with an off-chip oscillator, in order to generate a clock signal that gets distributed throughout a clocking network to the various components of the microprocessor. This on-chip clock generation circuitry is typically implemented using a ring oscillator comprised of an odd number of inverters linked together into a ring.
However, as microprocessor clock speeds continue to increase, the effects of noise in the power supply lines feeding into the ring oscillator introduce a significant amount of xe2x80x9cjitterxe2x80x9d into the resulting clock signal. This jitter gives rise to variations in the frequency of the clock signal that is delivered to different components within the microprocessor, and can thereby cause synchronization problems within the microprocessor chip.
The susceptibility of the clock signal to power supply noise can be reduced by replacing the ring oscillator with another type of clock generation circuit, such as an LC oscillator that is comprised of an inductor and a capacitor. Note that the frequency of an LC oscillator depends upon the inductance and capacitance of circuit elements that comprise the LC oscillator. Hence, an LC oscillator is relatively immune to fluctuations in power supply voltage.
However, providing an inductor on a semiconductor chip has so far not been practical due to large resistances through the metal traces that make up an inductor within a semiconductor chip. Fortunately, these large resistances tend to be less significant in comparison to the total impedance of the inductor at the higher frequencies at which on-chip clock generation circuits are beginning to operate. Furthermore, the use of copper instead of aluminum further reduces the resistance through the metal traces that make up an on-chip inductor.
Furthermore, clock generation circuits typically require some type of mechanism to adjust the frequency of the signal produced by the clock generation circuit. For an off-chip LC oscillator, this can be accomplished by adjusting the inductance of the LC circuit. However, there presently exists no mechanism for adjusting the inductance of an on-chip inductor.
What is needed is a method and an apparatus for adjusting the inductance of an on-chip inductor.
One embodiment of the present invention provides an inductor with a variable inductance within a semiconductor chip. This inductor includes a primary spiral composed of a conductive material embedded within the semiconductor chip to provide a source of variable inductance. It also includes a control spiral composed of the conductive material vertically displaced from the primary spiral in neighboring layers of the semiconductor chip. This control spiral is magnetically coupled with the primary spiral so that changing a control current through the control spiral induces a change in inductance through the primary spiral. The inductor also includes a controllable current source coupled to the control spiral that is configured to provide the control current.
One embodiment of the present invention includes a core surrounding the primary spiral and the control spiral in the semiconductor chip. This core is comprised of a core material with a magnetic permeability that facilitates magnetically coupling the control spiral with the primary spiral. In a variation on this embodiment, the core material includes a high frequency ferrite that operates at a frequency above one gigahertz without resistive eddy losses that substantially prevent a magnetic coupling between the control spiral and the primary spiral. In a variation on this embodiment, the high frequency ferrite can include NiZn.
In one embodiment of the present invention, the conductive material that makes up the primary spiral and the control spiral can include aluminum or copper.
In one embodiment of the present invention, the primary spiral includes a plurality of spirals composed of the conductive material that are vertically displaced from each other on different layers of the semiconductor chip, wherein the plurality of spirals are linked together through vertical conductors between the different layers of the semiconductor chip.
In one embodiment of the present invention, the primary spiral is in the form of a helix, wherein successive turns of the helix are vertically displaced from each other in different layers of the semiconductor chip.
In one embodiment of the present invention, successive turns of the primary spiral occupy the same layer of the semiconductor chip, but are radially displaced from prior turns of the primary spiral.
In one embodiment of the present invention, the controllable current source is configured to provide an alternating current at a frequency greater than one gigahertz with a controllable amplitude.
In one embodiment of the present invention, the controllable current source is configured to provide a direct current.
In one embodiment of the present invention, the controllable current source includes a control input for receiving a control voltage, wherein the control current produced by the controllable current source is proportionate to the control voltage.
One embodiment of the present invention provides an oscillator circuit within a semiconductor chip. This oscillator circuit includes an oscillator output with a first terminal and a second terminal. A number of circuit elements are coupled between the first terminal and the second terminal, including a capacitor, a negative resistance source, and a voltage-controlled inductor. This voltage-controlled inductor includes a primary spiral composed of a conductive material embedded within the semiconductor chip to provide a source of variable inductance, as well as a control spiral composed of the conductive material vertically displaced from the primary spiral in neighboring layers of the semiconductor chip. This control spiral is magnetically coupled with the primary spiral so that changing a control current through the control spiral induces a change in inductance through the primary spiral. The voltage-controlled inductor also includes a controllable current source coupled to the control spiral that is configured to provide the control current.